423 research outputs found
Chromospheric Inversions of a Micro-flaring Region
We use spectropolarimetric observations of the Ca II 8542~\AA\ line, taken
from the Swedish 1-m Solar Telescope (SST), in an attempt to recover dynamic
activity in a micro-flaring region near a sunspot via inversions. These
inversions show localized mean temperature enhancements of 1000~K in the
chromosphere and upper photosphere, along with co-spatial bi-directional
Doppler shifting of 5 - 10 km s. This heating also extends along a
nearby chromospheric fibril, co-spatial to 10 - 15 km s down-flows.
Strong magnetic flux cancellation is also apparent in one of the footpoints,
concentrated in the chromosphere. This event more closely resembles that of an
Ellerman Bomb (EB), though placed slightly higher in the atmosphere than is
typically observed.Comment: 9 pages, 9 figures, accepted in ApJ. Movies are stored here:
https://star.pst.qub.ac.uk/webdav/public/areid/Microflare
Analysis of roles and groups in blogosphere
In the paper different roles of users in social media, taking into
consideration their strength of influence and different degrees of
cooperativeness, are introduced. Such identified roles are used for the
analysis of characteristics of groups of strongly connected entities. The
different classes of groups, considering the distribution of roles of users
belonging to them, are presented and discussed.Comment: 8th International Conference on Computer Recognition Systems, CORES
201
Statistical Analysis of Small Ellerman Bomb Events
The properties of Ellerman bombs (EBs), small-scale brightenings in the
H-alpha line wings, have proved difficult to establish due to their size being
close to the spatial resolution of even the most advanced telescopes. Here, we
aim to infer the size and lifetime of EBs using high-resolution data of an
emerging active region collected using the Interferometric BIdimensional
Spectrometer (IBIS) and Rapid Oscillations of the Solar Atmosphere (ROSA)
instruments as well as the Helioseismic and Magnetic Imager (HMI) onboard the
Solar Dynamics Observatory (SDO). We develop an algorithm to track EBs through
their evolution, finding that EBs can often be much smaller (around 0.3") and
shorter lived (less than 1 minute) than previous estimates. A correlation
between G-band magnetic bright points and EBs is also found. Combining SDO/HMI
and G-band data gives a good proxy of the polarity for the vertical magnetic
field. It is found that EBs often occur both over regions of opposite polarity
flux and strong unipolar fields, possibly hinting at magnetic reconnection as a
driver of these events.The energetics of EB events is found to follow a
power-law distribution in the range of "nano-flare" (10^{22-25} ergs).Comment: 19 pages. 7 Figure
Ca II H and K Chromospheric Emission Lines in Late K and M Dwarfs
We have measured the profiles of the Ca II H and K chromospheric emission
lines in 147 main sequence stars of spectral type M5-K7 (0.30-0.55 solar
masses) using multiple high resolution spectra obtained during six years with
the HIRES spectrometer on the Keck 1 telescope. Remarkably, the average FWHM,
equivalent widths, and line luminosities of Ca II H and K increase by a factor
of 3 with increasing stellar mass over this small range of stellar masses. We
fit the H and K lines with a double Gaussian model to represent both the
chromospheric emission and the non-LTE central absorption. Most of the sample
stars display a central absorption that is typically redshifted by ~0.1 km/s
relative to the emission, but the nature of this velocity gradient remains
unknown. The FWHM of the H and K lines increase with stellar luminosity,
reminiscent of the Wilson-Bappu effect in FGK-type stars. Both the equivalent
widths and FWHM exhibit modest temporal variability in individual stars. At a
given value of M_v, stars exhibit a spread in both the equivalent width and
FWHM of Ca II H and K, due both to a spread in fundamental stellar parameters
including rotation rate, age, and possibly metallicity, and to the spread in
stellar mass at a given M_v. The K line is consistently wider than the H line,
as expected, and its central absorption is more redshifted, indicating that the
H and K lines form at slightly different heights in the chromosphere where the
velocities are slightly different. The equivalent width of H-alpha correlates
with Ca II H and K only for stars having Ca II equivalent widths above ~2
angstroms, suggesting the existence of a magnetic threshold above which the
lower and upper chromospheres become thermally coupled.Comment: 40 pages including 12 figures and 17 pages of tables, accepted for
publication in PAS
A New Multiple Stellar System in the Solar Neighborhood
Adaptive optics corrected images obtained with the CIAO instrument at the
Subaru 8.2-meter telescope show the presence of two subarsecond companions to
the nearby (d=19.3 pc) young star GJ 900, which was previously classified as a
single member of the IC 2391 supercluster. The two companions share the same
proper motion as the primary and are redder. Their projected separations from
the primary are 10 AU and 14.5 AU for B and C, respectively. The estimated
masses for the two new companions depend strongly on the age of the system. For
the range of ages found in the literature for IC 2391 supercluster members
(from 35 Myr to 200 Myr), the expected masses range from 0.2 M to 0.4
M for the B component, and from 0.09 M to 0.22 M for
the C component. The determination of the dynamical mass of the faintest
component of GJ 900 will yield the age of the system using theoretical
evolutionary tracks. The apparent separations of the GJ 900 system components
meet the observational criterion for an unstable Trapezium-type system, but
this could be a projection effect. Further observations are needed to establish
the nature of this interesting low-mass multiple system.Comment: Scheduled for publication in the Astronomical Journal (August 2003
Intensity enhancement of O VI ultraviolet emission lines in solar spectra due to opacity
Opacity is a property of many plasmas, and it is normally expected that if an
emission line in a plasma becomes optically thick, its intensity ratio to that
of another transition that remains optically thin should decrease. However,
radiative transfer calculations undertaken both by ourselves and others predict
that under certain conditions the intensity ratio of an optically thick to thin
line can show an increase over the optically thin value, indicating an
enhancement in the former. These conditions include the geometry of the
emitting plasma and its orientation to the observer. A similar effect can take
place between lines of differing optical depth. Previous observational studies
have focused on stellar point sources, and here we investigate the
spatially-resolved solar atmosphere using measurements of the I(1032 A)/I(1038
A) intensity ratio of O VI in several regions obtained with the Solar
Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on board the
Solar and Heliospheric Observatory (SoHO) satellite. We find several I(1032
A)/I(1038 A) ratios observed on the disk to be significantly larger than the
optically thin value of 2.0, providing the first detection (to our knowledge)
of intensity enhancement in the ratio arising from opacity effects in the solar
atmosphere. Agreement between observation and theory is excellent, and confirms
that the O VI emission originates from a slab-like geometry in the solar
atmosphere, rather than from cylindrical structures.Comment: 17 pages, 4 figures, ApJ Letters, in pres
Discovery of spatial periodicities in a coronal loop using automated edge-tracking algorithms
A new method for automated coronal loop tracking, in both spatial and temporal domains, is presented. Applying this technique to TRACE data, obtained using the 171 Ã… filter on 1998 July 14, we detect a coronal loop undergoing a 270 s kink-mode oscillation, as previously found by Aschwanden et al. However, we also detect flare-induced, and previously unnoticed, spatial periodicities on a scale of 3500 km, which occur along the coronal loop edge. Furthermore, we establish a reduction in oscillatory power for these spatial periodicities of 45% over a 222 s interval. We relate the reduction in detected oscillatory power to the physical damping of these loop-top oscillations
Solar Ellerman Bombs in 1D Radiative Hydrodynamics
Recent observations from the Interface Region Imaging Spectrograph (IRIS)
appear to show impulsive brightenings in high temperature lines, which when
combined with simultaneous ground based observations in H, appear
co-spatial to Ellerman Bombs (EBs). We use the RADYN 1-dimensional radiative
transfer code in an attempt to try and reproduce the observed line profiles and
simulate the atmospheric conditions of these events. Combined with the MULTI/RH
line synthesis codes, we compute the H, Ca II 8542~\AA, and Mg II h \&
k lines for these simulated events and compare them to previous observations.
Our findings hint that the presence of superheated regions in the photosphere
(10,000 K) is not a plausible explanation for the production of EB
signatures. While we are able to recreate EB-like line profiles in H,
Ca II 8542~\AA, and Mg II h \& k, we cannot achieve agreement with all of these
simultaneously.Comment: Accepted into ApJL. 4 Figures, 1 Tabl
Where Are The M Dwarf Disks Older Than 10 Million Years?
We present 11.7-micron observations of nine late-type dwarfs obtained at the
Keck I 10-meter telescope in December 2002 and April 2003. Our targets were
selected for their youth or apparent IRAS 12-micron excess. For all nine
sources, excess infrared emission is not detected. We find that stellar wind
drag can dominate the circumstellar grain removal and plausibly explain the
dearth of M Dwarf systems older than 10 Myr with currently detected infrared
excesses. We predict M dwarfs possess fractional infrared excess on the order
of L_{IR}/L_{*}\sim10^{-6} and this may be detectable with future efforts.Comment: 24 pages, 2 figures, accepted to Ap
- …